Process to manufacture low sulfur fuels

ABSTRACT

The instant invention relates to a process for producing high octane, low sulfur naphtha products through skeletal isomerization of feed olefins and hydrotreating.

CROSS REFERENCE TO RELATED APPLICATION

This application claims benefit of U.S. Provisional Patent ApplicationSer. No. 60/492,081 filed Aug. 1, 2003.

FIELD OF THE INVENTION

The instant invention relates to a process for upgrading hydrocarbonmixtures boiling within the naphtha range. More particularly, theinstant invention relates to a process to produce high octane, lowsulfur naphtha products through the skeletal isomerization of feedolefins and hydrotreating.

BACKGROUND OF THE INVENTION

Liquid hydrocarbon streams that boil within the naphtha range, i.e.below about 232° C., and produced from the Fluidized Catalytic CrackingUnit (“FCC”) are typically used as blending components for motorgasolines. Environmentally driven regulatory pressure concerning motorgasoline sulfur levels is expected to result in the widespreadproduction of less than 50 wppm sulfur mogas by the year 2004. Levelsbelow 10 wppm are being considered for later years in some regions ofthe world, and this will require deep desulfurization of naphthas inorder to conform to emission restrictions that are becoming morestringent. The majority, i.e., 90% or more, of sulfur contaminantspresent in motor gasolines are typically present in naphtha boilingrange hydrocarbon streams. However, the naphtha boiling range streamsare also rich in olefins, which boost octane, a desirable quality inmotor gasolines.

Thus, many processes have been developed to produce low sulfur productsfrom naphtha boiling range streams while attempting to minimize olefinloss, such as, for example, hydrodesulfurization processes. However,these processes also typically hydrogenate feed olefins to some degree,thus reducing the octane number of the product. Therefore, processeshave been developed that recover octane lost during desulfurization.Non-limiting examples of these processes can be found in U.S. Pat. Nos.5,298,150; 5,320,742; 5,326,462; 5,318,690; 5,360,532; 5,500,108;5,510,016; and 5,554,274, which are all incorporated herein byreference. In these processes, in order to obtain desirablehydrodesulfurization with a reduced octane loss, it is necessary tooperate in two steps. The first step is a hydrodesulfurization step, anda second step recovers octane lost during hydrodesulfurization.

Other processes have also been developed that seek to minimize octanelost during hydrodesulfurization. For example, selectivehydrodesulfurization is used to remove organically bound sulfur whileminimizing hydrogenation of olefins and octane reduction by varioustechniques, such as the use of selective catalysts and/or processconditions. For example, one selective hydrodesulfurization process,referred to as SCANfining, has been developed by ExxonMobil Research &Engineering Company in which olefinic naphthas are selectivelydesulfurized with little loss in octane. U.S. Pat. Nos. 5,985,136;6,013,598; and 6,126,814, all of which are incorporated by referenceherein, disclose various aspects of SCANfining. Although selectivehydrodesulfurization processes have been developed to avoid significantolefin saturation and loss of octane, such processes have a tendency toliberate H₂S a portion of which may react with retained olefins to formmercaptan sulfur by reversion.

Thus, there still exists a need in the art for a process to reduce thesulfur content in naphtha boiling range hydrocarbon streams whileminimizing octane loss and mercaptan reversion.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows research octane versus desulfurization results from theexample.

FIG. 2 shows iso-olefin to n-olefin ratio results from the example.

FIG. 3 shows iso-paraffin to n-paraffin ratio results from the example.

SUMMARY OF THE INVENTION

The instant invention is directed at a process for producing low sulfurnaphtha products. The process comprises:

-   -   a) contacting a naphtha boiling range feedstream containing        organically bound sulfur and olefins in a first reaction zone        operated under effective isomerization conditions and in the        presence of hydrogen-containing treat gas with a first catalyst        selected from medium pore zeolites to produce a first reaction        zone effluent; and    -   b) hydrotreating at least a portion of the first reaction zone        effluent of step a) above in a second reaction zone operated        under effective hydrotreating conditions and in the presence of        hydrogen-containing treat gas and a second catalyst selected        from hydrotreating catalysts comprising about 0.1 to 27 wt. % of        at least one Group VIII metal oxide and about 1 to 45 wt. % of        at least one Group VI metal oxide to produce a desulfurized        product.

DETAILED DESCRIPTION OF THE INSTANT INVENTION

It should be noted that the terms “hydrotreating” and“hydrodesulfurization” are sometimes used interchangeably herein, andthe prefixes “i-” and “n” are meant to refer to “iso-” and “normal”,respectively.

In the hydrotreating of naphtha boiling range feedstreams, olefins aretypically saturated in the hydrotreating zone resulting in a decrease inoctane number of the desulfurized product. However, the presentinvention reduces the decrease in octane of the desulfurized productthrough the use of a novel process involving contacting a naphthaboiling range feedstream in a first reaction zone operated undereffective isomerization conditions. This first reaction zone contains afirst catalyst selected from medium pore zeolites, and the naphthaboiling range feedstream is contacted with the first catalyst in thepresence of a hydrogen-containing treat gas. The contacting of thenaphtha boiling range feedstream with the first catalyst produces afirst reaction zone effluent. The first reaction zone effluent is thencontacted in a second reaction zone operated under effectivehydrotreating conditions, and in the presence of hydrogen-containingtreat gas, with a second catalyst comprising at least one Group VIIImetal oxide and at least one Group VI metal oxide supported on asuitable substrate.

The desulfurized product thus obtained has a higher iso-paraffin ton-paraffin ratio, and thus a higher octane than a desulfurized naphthatreated by a selective or non-selective hydrotreating process only,i.e., without an octane recovery step. The higher octane of thedesulfurized product results from the unexpected finding by theinventors hereof that by operating the first reaction zone underconditions effective for encouraging the skeletal isomerization ofn-olefins to iso-olefins results in a desulfurized naphtha producthaving a higher octane number than a desulfurized product resulting froma selective hydrodesulfurization process only. The inventors hereof havefound that the degree of skeletal isomerization of n-olefins toiso-olefins benefits the final product because the saturation ofiso-olefins to iso-paraffins that occurs in the second reaction zoneherein provides for less octane loss in the final product when comparedto the saturation of n-olefins to n-paraffins. It should be noted thatiso-paraffins typically have a much higher octane than theircorresponding n-paraffin. Further, the rate of saturation of iso-olefinsis typically slower than that of n-olefins. Therefore, by increasing theratio of iso-olefins to n-olefins present in the first reaction zoneeffluent, the resulting desulfurized naphtha product exiting the secondreaction zone also has a higher iso-olefin to n-olefin ratio as well asa higher olefin content, and thus a higher octane than a desulfurizednaphtha treated by a selective or non-selective hydrotreating processonly.

In the hydroprocessing of naphtha boiling range hydrocarbon feedstreams,it is typically highly desirable to remove sulfur-containing compoundsfrom the naphtha boiling range feedstreams with as little olefinsaturation as possible. It is also highly desirable to convert as muchof the organic sulfur species of the naphtha to hydrogen sulfide with aslittle mercaptan reversion as possible. By mercaptan reversion we meanthe reaction of hydrogen sulfide with olefins during the hydrotreatingto form undesirable alkylmercaptans. The inventors hereof haveunexpectedly found that through the use of the presently claimedinvention, high levels of sulfur can be removed from an olefinic naphthastream without excessive olefin saturation or mercaptan reversion takingplace.

Feedstreams suitable for use in the present invention include naphthaboiling range refinery streams that typically boil in the range of about50° F. (10° C.) to about 450° F. (232° C.) containing olefins as well assulfur-containing compounds. Thus, the term “naphtha boiling rangefeedstream” as used herein includes those streams having an olefincontent of at least about 5 wt. %. Non-limiting examples of naphthaboiling range feedstreams that can be treated by the present inventioninclude fluid catalytic cracking unit naphtha (FCC catalytic naphtha orcat naphtha), steam cracked naphtha, and coker naphtha. Also includedare blends of olefinic naphthas with non-olefinic naphthas as long asthe blend has an olefin content of at least about 5 wt. %, based on thetotal weight of the naphtha feedstream.

Cracked naphtha refinery streams generally contain not only paraffins,naphthenes, and aromatics, but also unsaturates, such as open-chain andcyclic olefins, dienes, and cyclic hydrocarbons with olefinic sidechains. The olefin-containing naphtha feedstream can contain an overallolefins concentration ranging as high as about 70 wt. %, more typicallyas high as about 60 wt. %, and most typically from about 5 wt. % toabout 40 wt. %. The olefin-containing naphtha feedstream can also have adiene concentration up to about 15 wt. %, but more typically less thanabout 5 wt. % based on the total weight of the feedstock. The sulfurcontent of the naphtha feedstream will generally range from about 50wppm to about 7000 wppm, more typically from about 100 wppm to about5000 wppm, and most typically from about 100 to about 3000 wppm. Thesulfur will usually be present as organically bound sulfur. That is, assulfur compounds such as simple aliphatic, naphthenic, and aromaticmercaptans, sulfides, di- and polysulfides and the like. Otherorganically bound sulfur compounds include the class of heterocyclicsulfur compounds such as thiophene, tetrahydrothiophene, benzothiopheneand their higher homologs and analogs. Nitrogen can also be present in arange from about 5 wppm to about 500 wppm.

The feedstreams used herein are typically preheated prior to enteringthe first reaction zone herein and final heating is typically targetedto the effective hydrotreating temperatures in the second reaction zone.If the naphtha boiling range feedstream is preheated, it can be reactedwith the hydrogen-containing treat gas stream prior to, during, and/orafter preheating. At least a portion of the hydrogen-containing treatgas can also be added at an intermediate location in the first reactionzone. Hydrogen-containing treat gasses suitable for use in the presentlydisclosed process can be comprised of substantially pure hydrogen or canbe mixtures of other components typically found in refinery hydrogenstreams. It is preferred that the hydrogen-containing treat gas streamcontains little, more preferably no, hydrogen sulfide. Thehydrogen-containing treat gas purity should be at least about 50% byvolume hydrogen, preferably at least about 75% by volume hydrogen, andmore preferably at least about 90% by volume hydrogen for best results.It is most preferred that the hydrogen-containing stream besubstantially pure hydrogen.

In the first reaction zone, the above-described naphtha boiling rangefeedstream is contacted with a first catalyst comprising a medium porezeolite. Zeolites are porous crystalline materials, and medium porezeolites as used herein can be any zeolite described as a medium porezeolite in Atlas of Zeolite Structure Types, W. M. Maier and D. H.Olson, Butterworths. Typically, medium pore zeolites are defined asthose having a pore size of about 5 to about 7 Angstroms, such that thezeolite freely sorbs molecules such as n-hexane, 3-methylpentane,benzene and p-xylene. Another common classification used for medium porezeolites involves the Constraint Index test which is described in U.S.Pat. No. 4,016,218, which is hereby incorporated by reference. Mediumpore zeolites typically have a Constraint Index of about 1 to about 12,based on the zeolite alone without modifiers and prior to treatment toadjust the diffusivity of the catalyst. Preferred medium pore zeolitesfor use herein are selected from the group consisting of ZSM-23 andZSM-48 with ZSM-48 being the most preferred.

The medium pore zeolite used as the first catalyst may be combined witha suitable porous binder or matrix material. Non-limiting examples ofsuch materials include active and inactive materials such as clays,silica, and/or metal oxides such as alumina. Non-limiting examples ofnaturally occurring clays that can be composited include clays from themontmorillonite and kaolin families including the subbentonites, and thekaolins commonly known as Dixie, McNamee, Georgia, and Florida clays.Others in which the main mineral constituent is halloysite, kaolinite,dickite, nacrite, or anauxite may also be used. The clays can be used inthe raw state as originally mixed or subjected to calcination, acidtreatment, or chemical modification prior to being combined with themedium pore zeolite.

It is preferred that the porous matrix or binder material comprisessilica, alumina, or a kaolin clay. It is more preferred that the bindermaterial comprise alumina. In this embodiment the alumina is present ina ratio of less than about 15 parts zeolite to one part binder,preferably less than about 10, more preferably less than about 5, andmost preferably about 2.

The first reaction zone can be comprised of one or more fixed bedreactors or reaction zones each of which can comprise one or morecatalyst beds of the same first catalyst. Although other types ofcatalyst beds can be used, fixed beds are preferred. Such other types ofcatalyst beds include fluidized beds, ebullating beds, slurry beds, andmoving beds. Interstage cooling or heating between reactors, or betweencatalyst beds in the same reactor, can be employed since some olefinsaturation can take place, and olefin saturation and the desulfurizationreaction are generally exothermic. A portion of the heat generatedduring hydrotreating can be recovered. Where this heat recovery optionis not available, conventional cooling may be performed through coolingutilities such as cooling water or air, or through use of a hydrogenquench stream. In this manner, optimum reaction temperatures can be moreeasily maintained.

As stated above, the above-defined first catalyst is placed in a firstreaction zone that is operated under effective isomerization conditions.By effective isomerization conditions, it is meant those conditions thatprovide for the skeletal isomerization of at least about 20 wt. % of then-olefins present in the feedstream to iso-olefins, preferably at leastabout 40 wt. %, more preferably at least about 50 wt. %. By skeletalisomerization, it is meant the reorientation of the molecular structureof the normal olefins (n-olefins) with a preference for branched chainiso-olefins over straight. Thus, skeletal isomerization, as used herein,refers to the conversion of a normal olefin to a branched olefin or tothe rearranging or moving of branch carbon groups, which are attached tothe straight chain olefin molecule, to a different carbon atom, andnon-skeletal isomerization can be described as the rearranging of theposition of the double bond within the straight chain or branched olefinmolecule. These conditions typically include temperatures ranging fromabout 150° C. to about 425° C., preferably about 200° C. to about 370°C., more preferably about 230° C. to about 350° C. Typical weight hourlyspace velocities (“WHSV”) range from about 0.1 to about 20 hr⁻¹,preferably from about 0.5 to about 5 hr⁻¹. Any effective pressure can beutilized, and pressures typically range from about 4 to about 70atmospheres, preferably from about 10 to about 40 atmospheres.

The contacting of the naphtha boiling-range feedstream with the firstcatalyst under effective isomerization conditions produces a firstreaction zone effluent. At least a portion of the first reaction zoneeffluent, preferably substantially all of the first reaction zoneeffluent, is then passed to a second reaction zone wherein the firstreaction zone effluent is contacted in a second reaction zone with ahydrotreating catalyst in the presence of a hydrogen-containing treatgas under effective hydrotreating conditions. The second reaction zonecan also be comprised of one or more fixed bed reactors or reactionzones each of which can comprise one or more catalyst beds of the samecatalyst. Although other types of catalyst beds can be used,non-limiting examples of suitable bed types include fluidized beds,ebullating beds, slurry beds, and moving beds. Preferred are fixedcatalyst beds and it is more preferred that the first reaction zone andthe second reaction zone be in the same reaction vessel. Interstagecooling or heating between reactors or reaction zones, or betweencatalyst beds in the same reactor, can be employed since some olefinsaturation can take place, and olefin saturation and the desulfurizationreaction are generally exothermic. A portion of the heat generatedduring hydrodesulfurization can be recovered. Where this heat recoveryoption is not available, conventional cooling may be performed throughcooling utilities such as cooling water or air, or through use of ahydrogen quench stream. In this manner, optimum reaction temperaturescan be more easily maintained.

Suitable second reaction zone catalysts are those that are comprised ofat least one Group VIII metal oxide, preferably an oxide of a metalselected from Fe, Co and Ni, more preferably Co and/or Ni, and mostpreferably Co; and at least one Group VI metal oxide, preferably anoxide of a metal selected from Mo and W, more preferably Mo, on a highsurface area support material, preferably alumina. Other suitable secondreaction zone catalysts include zeolitic catalysts, as well as noblemetal catalysts where the noble metal is selected from Pd and Pt. It iswithin the scope of the present invention that more than one type ofcatalyst be used in the same reaction vessel. The at least one GroupVIII metal oxide of the second reaction zone catalysts is typicallypresent in an amount ranging from about 2 to about 20 wt. %, preferablyfrom about 4 to about 12%. The at least one Group VI metal oxide willtypically be present in an amount ranging from about 1 to about 50 wt.%, preferably from about 1 to about 10 wt. %, and more preferably fromabout 1 to about 5 wt. %. All metal oxide weight percents are onsupport. By “on support” we mean that the percents are based on theweight of the support. For example, if the support were to weigh 100 g.then 20 wt. % Group VIII metal oxide would mean that 20 g. of Group VIIImetal oxide was on the support.

Preferred catalysts of the second reaction zone will also have a highdegree of metal sulfide edge plane area as measured by the OxygenChemisorption Test described in “Structure and Properties of MolybdenumSulfide: Correlation of O₂ Chemisorption with HydrodesulfurizationActivity,” S. J. Tauster et al., Journal of Catalysis 63, pp 515-519(1980), which is incorporated herein by reference. The OxygenChemisorption Test involves edge-plane area measurements made whereinpulses of oxygen are added to a carrier gas stream and thus rapidlytraverse the catalyst bed. For example, the oxygen chemisorption will befrom about 800 to 2,800, preferably from about 1,000 to 2,200, and morepreferably from about 1,200 to 2,000 μmol oxygen/gram MoO₃.

The most preferred catalysts for the second reaction zone can becharacterized by the properties: (a) a MoO₃ concentration of about 1 to25 wt. %, preferably about 2 to 10 wt. %, and more preferably about 3 to6 wt. %, based on the total weight of the catalyst; (b) a CoOconcentration of about 0.1 to 6 wt. %, preferably about 0.5 to 5 wt. %,and more preferably about 1 to 3 wt. %, also based on the total weightof the catalyst; (c) a Co/Mo atomic ratio of about 0.1 to about 1.0,preferably from about 0.20 to about 0.80, more preferably from about0.25 to about 0.72; (d) a median pore diameter of about 60 Å to about200 Å, preferably from about 75 Å to about 175 Å, and more preferablyfrom about 80 Å to about 150 Å; (e) a MoO₃ surface concentration ofabout 0.5×10⁻⁴ to about 3×10⁻⁴ g. MoO₃/m², preferably about 0.75×10⁻⁴ toabout 2.5×10⁻⁴, more preferably from about 1×10 ⁻⁴ to 2×10⁻⁴; and (f) anaverage particle size diameter of less than 2.0 mm, preferably less thanabout 1.6 mm, more preferably less than about 1.4 mm, and mostpreferably as small as practical for a commercial hydrodesulfurizationprocess unit.

The catalysts used in the second reaction zone of the present inventionare preferably supported catalysts. Any suitable refractory catalystsupport material, preferably inorganic oxide support materials may beused as supports for the catalyst of the present invention. Non-limitingexamples of suitable support materials include: zeolites, alumina,silica, titania, calcium oxide, strontium oxide, barium oxide, carbons,zirconia, diatomaceous earth, lanthanide oxides including cerium oxide,lanthanum oxide, neodymium oxide, yttrium oxide, and praseodymium oxide;chromia, thorium oxide, urania, niobia, tantala, tin oxide, zinc oxide,and aluminum phosphate. Preferred are alumina, silica, andsilica-alumina. More preferred is alumina. Magnesia can also be used forthe second reaction zone catalysts. It is to be understood that thesupport material can also contain small amounts of contaminants, such asFe, sulfates, silica, and various metal oxides that can be introducedduring the preparation of the support material. These contaminants arepresent in the raw materials used to prepare the support and willpreferably be present in amounts less than about 1 wt. %, based on thetotal weight of the support. It is more preferred that the supportmaterial be substantially free of such contaminants. It is an embodimentof the present invention that about 0 to 5 wt. %, preferably from about0.5 to 4 wt. %, and more preferably from about 1 to 3 wt. %, of anadditive be present in the support, which additive is selected from thegroup consisting of phosphorus and metals or metal oxides from Group IA(alkali metals) of the Periodic Table of the Elements.

As previously stated, the first reaction zone effluent is contacted withthe above-defined second catalyst in a second reaction zone undereffective hydrotreating conditions to produce a desulfurized product. Byeffective hydrotreating conditions, it is meant those conditions chosenthat will achieve a resulting desulfurized naphtha product having lessthan 100 wppm sulfur, preferably less than 50 wppm sulfur, morepreferably less than 30 wppm sulfur. These conditions typically includetemperatures ranging from about 150° C. to about 425° C., preferablyabout 200° C. to about 370° C., more preferably about 230° C. to about350° C. Typical weight hourly space velocities (“WHSV”) range from about0.1 to about 20 hr⁻¹, preferably from about 0.5 to about 5 hr⁻¹. Anyeffective pressure can be utilized, and pressures typically range fromabout 4 to about 70 atmospheres, preferably 10 to 40 atmospheres. Itshould be noted that although the range of operating conditions for thesecond reaction zone is similar to that for the first reaction zone,both reaction zones could operate under different conditions. In a mostpreferred embodiment, the effective hydrotreating conditions areselective hydrotreating conditions configured to achieve a sulfur levelwithin the above-defined sulfur ranges, most preferably the desulfurizednaphtha product has a sulfur level sufficiently low to meet currentregulatory standards in place at that time. By selective hydrotreatingconditions, it is meant conditions such as those contained in U.S. Pat.Nos. 5,985,136; 6,013,598; and 6,126,814, all of which have already beenincorporated by reference herein, which disclose various aspects ofSCANfining, a process developed by the ExxonMobil Research andEngineering Company in which olefinic naphthas are selectivelydesulfurized with little loss in octane.

As previously stated, the desulfurized product thus obtained will have ahigher iso-paraffin to n-paraffin ratio, and thus a higher octane than adesulfurized naphtha treated by a selective or non-selectivehydrotreating process. Typical iso-paraffin to n-paraffin ratios in thedesulfurized product resulting from the present process are typicallygreater than about 1, preferably about 2, more preferably about 3. Thus,compared to selective hydrodesulfurizafion processes, the presentlyclaimed process produces desulfurized naphtha products with a higheroctane at constant olefin saturation even when both processes maintainsimilar desulfurization/olefin saturation selectivity.

The above description is directed to several embodiments of the presentinvention. Those skilled in the art will recognize that otherembodiments that are equally effective could be devised for carrying outthe spirit of this invention.

The following example will illustrate the improved effectiveness of thepresent invention, but is not meant to limit the present invention inany fashion.

EXAMPLE

An FCC naphtha was treated with acidic materials (Amberlyst-15 andalumina) to remove nitrogen-containing compounds. The naphtha feedhaving a reduced amount of nitrogen compounds was used in the presentexample, and its properties are outlined in Table 1 below. TABLE 1 APIGravity 56 Sulfur 606 wppm Nitrogen  1 wppm Bromine Number 72 ResearchOctane Number 92.1 N-Paraffins  3.22 wt. % I-Paraffins 23.22 wt. %Naphthenes  8.38 wt. % Aromatics 29.69 wt % N-Olefins 11.95 wt. %I-Olefins 17.35 wt. % Other Olefins  6.20 wt. % Distillation ASTM D228710%  42° C. 30%  79° C. 50% 109° C. 70% 138° C. 90% 174° C.

The feed described in Table 1 above was then subjected to twoside-by-side experiments to demonstrate the concept of olefinisomerization/desulfurization to preserve octane of the desulfurizednaphtha product. These experiments were conducted in identicaldown-flow, fixed-bed pilot units that share a common sand bath forcontrol of reactor temperature.

In these experiments, two units, Unit A and Unit B were used to evaluatethe effectiveness of the present invention. Unit A utilized a stackedbed configuration and Unit B used a single bed. The catalyst loadings ofUnit A were 2.5 cc of ZSM-48 as the first catalyst in the first reactionzone and 2.5 cc of a catalyst comprising 4.3 wt. % MoO₃, 1.2 wt. % CoO,on alumina with a median pore diameter of 95 Å was used as the secondcatalyst in the second reaction zone. Unit B utilized 2.5 cc of acatalyst comprising 4.3 wt. % MoO₃, 1.2 wt. % CoO, on alumina with amedian pore diameter of 95 Å only.

The feed was contacted with the catalyst(s) system contained in bothUnit A and Unit B under the same conditions. These conditions included aflow rate of 10 cc/hr, a hydrogen treat gas rate of 59.4 cc/min ofsubstantially pure hydrogen, and a total system pressure of 1.84 MPa.The reactor temperature (sand bath) was varied from 250° C. to 290° C.The results of the two experiments were then evaluated and are containedin FIGS. 1, 2, and 3 below. Based on the results contained in FIGS. 1, 2and 3, the catalyst system of the instant invention saves octane becausethe products resulting from treating a naphtha boiling range feed streamwith the present process unexpectedly have more branched olefins andparaffins.

FIG. 1 shows that at constant desulfurization, the stacked bed system ofUnit A produced a product having a higher research octane number thanthe catalyst system of Unit B.

FIG. 2 shows that at constant olefin saturation, the stacked bedcatalyst system of Unit A gave a higher iso-olefin to n-olefin ratio inthe first reaction zone effluent than the catalyst system of Unit B. Theolefin saturation is expressed as a reduction of bromine number (HDBr),which is directly related to the olefin content. The reduction inbromine number was measured according to ASTM 1159

FIG. 3 shows that at constant olefin saturation, the stacked bedcatalyst system of Unit A produced a product having a higheriso-paraffin to n-paraffin ratio that the catalyst system of Unit B.

1. A process for producing low sulfur naphtha products from an olefinand sulfur containing naphtha boiling range feedstream comprising: a)contacting a naphtha boiling range feedstream containing organicallybound sulfur and olefins in a first reaction zone operated undereffective isomerization conditions and in the presence ofhydrogen-containing treat gas with a first catalyst selected from mediumpore zeolites to produce a first reaction zone effluent; and b)hydrotreating at least a portion of the first reaction zone effluent ofstep a) above in a second reaction zone operated under effectivehydrotreating conditions and in the presence of hydrogen-containingtreat gas and a second catalyst selected from hydrotreating catalystscomprising about 2 to 20 wt. % of at least one Group VIII metal oxideand about 1 to 50 wt. % of at least one Group VI metal oxide to producea desulfurized product.
 2. The process according to claim 1 wherein saidfirst and second reaction zones comprise one or more catalyst bedsselected from the group consisting of fluidized beds, ebullating beds,slurry beds, fixed beds, and moving beds wherein each of said one ormore catalyst beds contains a catalyst suitable for the reaction zone inwhich the catalyst bed is located.
 3. The process according to claim 2wherein said first and second reaction zones are located in the samereaction vessel.
 4. The process according to claim 3 wherein said firstand second reaction zones comprise one or more fixed catalyst beds. 5.The process according to claim 2 wherein said process further comprisesinterstage cooling between said first and second reaction zone, orbetween catalyst beds in said first and second reaction zone.
 6. Theprocess according to claim 2 wherein said first catalyst is selectedfrom ZSM-23 and ZSM-48.
 7. The process according to claim 2 wherein saidfirst catalyst is ZSM-48.
 8. The process according to claim 6 whereinsaid second catalyst is a hydrotreating catalyst comprising about 4 toabout 12% of a Group VIII metal oxide and about 10 to about 40 wt. % ofa Group VI metal oxide.
 9. The process according to claim 6 wherein saidsecond catalyst is a hydrotreating catalyst comprising about 1 to 25 wt.% MoO₃, about 0.1 to 6 wt. % CoO wherein said CoO and MoO₃ are presentin an atomic ratio of about 0.1 to about 1.0 Co/Mo, and said catalysthas a median pore diameter of about 75 Å to about 175 Å, wherein saidsecond catalyst has a MoO₃ surface concentration of about 0.5×10⁻⁴ toabout 3×10⁻⁴ g and an average particle size diameter of less than 2.0mm.
 10. The process according to claim 1 wherein said effectiveisomerization conditions are selected to cause skeletal isomerization ofat least about 20 wt. % of the n-olefins present in said naphtha boilingrange feedstream.
 11. The process according to claim 10 wherein saidfirst reaction zone effluent has a higher ratio of iso-olefins ton-olefins than the naphtha boiling range feedstream.
 12. The processaccording to claim 9 wherein said second catalyst further comprises asuitable binder or matrix material selected from zeolites, alumina,silica, titania, calcium oxide, strontium oxide, barium oxide, carbons,zirconia, diatomaceous earth, lanthanide oxides including cerium oxide,lanthanum oxide, neodymium oxide, yttrium oxide, and praseodymium oxide;chromia, thorium oxide, urania, niobia, tantala, tin oxide, zinc oxide,and aluminum phosphate.
 13. The process according to claim 12 whereinsaid suitable binder or matrix support of said second catalyst alsocontains less than about 1 wt. % of contaminants, such as Fe, sulfates,silica, and various metal oxides that can be introduced during thepreparation of the support.
 14. The process according to claim 13wherein said suitable binder or matrix support of said second catalystalso contains about 0 to 5 wt. % of an additive selected from the groupconsisting of phosphorus and metals or metal oxides from Group IA(alkali metals) of the Periodic Table of the Elements.
 15. The processaccording to claim 12 wherein said suitable binder or matrix material isselected from alumina, silica, and silica-alumina.
 16. The processaccording to claim 12 wherein said suitable binder or matrix material isalumina.
 17. The process according to claim 10 wherein said firstcatalyst further comprises a suitable porous binder or matrix materialselected from clays, silica, and/or metal oxides such as alumina. 18.The process according to claim 17 wherein said suitable porous binder ormatrix material is selected from silica, alumina, or a kaolin clay. 19.The process according to claim 16 wherein said suitable porous binder ormatrix material is alumina present in a ratio of less than about 15parts zeolite to one part binder.
 20. The process according to claim 19wherein said effective hydrotreating conditions are selected in such amanner that said desulfurized naphtha product has a sulfur level lessthan 100 wppm sulfur.
 21. The process according to claim 20 wherein saideffective hydrotreating conditions are selective hydrotreatingconditions.
 22. The process according to claim 20 wherein saiddesulfurized naphtha product has a higher concentration of iso-paraffinsthan n-paraffins.
 23. The process according to claim 22 wherein thenaphtha boiling range feedstream containing organically bound sulfur andolefins is preheated prior to entering said first reaction zone.
 24. Aprocess for producing low sulfur naphtha products from an olefin andsulfur containing naphtha boiling range feedstream comprising: a)contacting a naphtha boiling range feedstream containing organicallybound sulfur and olefins in a first reaction zone operated undereffective isomerization conditions and in the presence ofhydrogen-containing treat gas with a first catalyst selected from ZSM-23and ZSM-48 to produce a first reaction zone effluent having a higherratio of iso-olefins to n-olefins than the naphtha boiling rangefeedstream; and b) hydrotreating at least a portion of the firstreaction zone effluent of step a) above in a second reaction zoneoperated under effective hydrotreating conditions and in the presence ofhydrogen-containing treat gas and a second catalyst selected fromhydrotreating catalysts comprising about 1 to 25 wt. % MoO₃ about 0.1 to6 wt. % CoO wherein said CoO and MoO₃ are present in an atomic ratio ofabout 0.1 to about 1.0 Co/Mo, and said catalyst has a median porediameter of about 75 Å to about 175 Å, wherein said second catalyst hasa MoO₃ surface concentration of about 0.75×10⁻⁴ to about 2.5×10⁻⁴ g andan average particle size diameter of less than 2.0 mm to produce adesulfurized product having a sulfur level less than 100 wppm sulfur anda higher concentration of iso-paraffins than n-paraffins.
 25. Theprocess according to claim 24 wherein said first catalyst is ZSM-48. 26.The process according to claim 25 wherein said second catalyst is ahydrotreating catalyst comprising about 4 to 19 wt. % MoO₃, about 0.5 to5.5 wt. % CoO wherein said CoO and MoO₃ are present in an atomic ratioof about 0.20 to about 0.80 Co/Mo, and said catalyst has a median porediameter of about 75 Å to about 175 Å, wherein said second catalyst hasa MoO₃ surface concentration of about 0.5×10⁴ to about 3×10⁴ g and anaverage particle size diameter of less than 1.6 mm.
 27. The processaccording to claim 26 wherein said effective isomerization conditionsare selected to cause skeletal isomerization of at least about 20 wt. %of the n-olefins present in said naphtha boiling range feedstream. 28.The process according to claim 26 wherein said second catalyst furthercomprises a suitable binder or matrix material selected from zeolites,alumina, silica, titania, calcium oxide, strontium oxide, barium oxide,carbons, zirconia, diatomaceous earth, lanthanide oxides includingcerium oxide, lanthanum oxide, neodymium oxide, yttrium oxide, andpraseodymium oxide; chromia, thorium oxide, urania, niobia, tantala, tinoxide, zinc oxide, and aluminum phosphate.
 29. The process according toclaim 28 wherein said suitable binder or matrix material is selectedfrom alumina, silica, and silica-alumina.
 30. The process according toclaim 24 wherein said first catalyst further comprises a suitable porousbinder or matrix material selected from clays, silica, and/or metaloxides such as alumina.
 31. The process according to claim 30 whereinsaid first reaction zone catalyst further comprises suitable porousbinder or matrix material selected from silica, alumina, or a kaolinclay.
 32. The process according to claim 31 wherein said effectivehydrotreating conditions are selected in such a manner that saiddesulfurized naphtha product has a sulfur level less than 100 wppmsulfur.
 33. The process according to claim 32 wherein said secondeffective hydrotreating conditions are selective hydrotreatingconditions.
 34. A process for producing low sulfur naphtha products froman olefin and sulfur containing naphtha boiling range feedstreamcomprising: a) contacting a naphtha boiling range feedstream containingorganically bound sulfur and olefins in a first reaction zone operatedunder effective isomerization conditions selected to cause skeletalisomerization of at least about 20 wt. % of the n-olefins contained insaid feedstream, and in the presence of hydrogen-containing treat gas,with a first catalyst comprising ZSM-48 and an alumina binder, whereinsaid binder and ZSM-48 are present in a ratio of less than about 15parts zeolite to one part binder, to produce a first reaction zoneeffluent having a higher ratio of iso-olefins to n-olefins than thenaphtha boiling range feedstream; and b) hydrotreating the firstreaction zone effluent of step a) above in a second reaction zoneoperated under selective hydrotreating conditions and in the presence ofhydrogen-containing treat gas and a second catalyst selected fromhydrotreating catalysts comprising about 5 to 16 wt. % MoO₃, about 1 to5 wt. % CoO wherein said CoO and MoO₃ are present in an atomic ratio ofabout 0.25 to about 0.72 Co/Mo, and said catalyst has a median porediameter of about 80 Å to about 150 Å, wherein said second catalyst hasa MoO₃ surface concentration of about 1×10⁻⁴ to 2×10⁻⁴ g and an averageparticle size diameter of less than 1.4 mm to produce a desulfurizedproduct having a sulfur level less than 50 wppm sulfur and a higherconcentration of iso-paraffins than n-paraffins.